Nitrogen oxide sensor with attenuated oxygen dependence of the nox signal

ABSTRACT

In a method and with a circuit for operating a nitrogen oxide sensor for determining the nitrogen oxide (NOx) concentration in a gas mixture, in particular in post-treatment of an automotive exhaust, an electric pumping voltage (U_APE,IPE) which induces a pumping current (I_pump) being applied between an internal pump electrode (IPE) and an external pump electrode (APE) of a pumping cell, by which a constant oxygen partial pressure is established in a first test gas space by pumping oxygen in or out, the pumping voltage (U_APE,IPE) being regulated so that a constant voltage value is established at electrodes of a concentration cell, a NOx-sensitive third electrode situated in a second test gas space being operated as the second pumping cell in which a limit pumping current is established, indicating the NOx concentration, in order to minimize the influence of oxygen on the nitrogen oxide signal measured, the pumping current (I_pump) is switched off or reduced in a controlled manner within a measurement time window (T_Mess) and the NOx concentration is determined within the measurement time window (T_Mess).

FIELD OF THE INVENTION

The present invention relates in general to exhaust-gas aftertreatment,in particular using lambda regulation in fuel-driven motor vehicles, andin particular to a method and a circuit for operating a nitrogen oxidesensor suitable for use with such a lambda regulation for determiningthe nitrogen oxide concentration in an exhaust-gas mixture.

BACKGROUND INFORMATION

Lambda regulation in combination with a catalytic converter is the mosteffective method today for purifying the exhaust from an internalcombustion engine. Very low emission levels are achievable only bycombination with the ignition and injection systems available today.Using a three-way or selective catalytic converter is particularlyeffective. This type of catalytic converter is capable of achievinggreater than 98% degradation of hydrocarbons, carbon monoxide, andnitrogen oxides if the engine is operated at LAMBDA=1 in a range ofapproximately 1% around the stoichiometric air/fuel ratio. LAMBDA is anindicator of the deviation of the actual air/fuel mixture from LAMBDA=1,corresponding to a weight ratio of 14.7 kg/air to 1 kg/gasoline, asrequired theoretically for complete combustion, i.e., LAMBDA is theratio of air mass supplied to theoretical air demand.

In lambda regulation, a measurement is performed on the particularexhaust and the quantity of fuel supplied is corrected immediatelyaccording to the results of this measurement via the injection system,for example. The measuring sensor is a lambda probe which shows a suddenchange in voltage at precisely LAMBDA=1 and then delivers a signalindicating whether the mixture is richer or leaner than LAMBDA=1. Theefficiency of the lambda probe is based on the principle of a galvanicoxygen concentration cell having a solid-state electrolyte.

Known lambda probes are two-point probes which operate by the Nernstprinciple using a Nernst cell. The solid-state electrolyte is composedof two interfaces separated by a ceramic. The ceramic material usedbecomes conducting for oxygen ions at approximately 350° C., so a Nernstvoltage is generated when different oxygen levels prevail on the twosides of the ceramic between the interfaces. This voltage is a measureof the difference in oxygen levels on the two sides of the ceramic.Since the residual oxygen content of the exhaust of an internalcombustion engine depends greatly on the air/fuel ratio the mixturesupplied to the engine, it is possible to use the oxygen content of theexhaust as a measure of the actual prevailing air/fuel ratio.

With broadband probes, the measuring sensor is designed as a broadbandsensor having solid electrolyte layers and a plurality of electrodes.Such a design is described in German Published Patent Application No.199 12 102, in particular on pages 8 and 9 and in FIG. 1, to whichreference is made to the full extent in the present context. Theseelectrodes are also diagramed schematically in FIG. 1, which isdescribed in detail below. A portion of these electrodes forms a pumpingcell in this sensor, while another portion forms what is called aconcentration cell. Furthermore, a first test gas space is formed by thesolid electrolyte layers.

A pumping voltage U_APE,IPE is applied to the electrodes of the pumpingcell (FIG. 1); a constant oxygen partial pressure is established in thefirst test gas space by pumping oxygen in or out using this pumpingvoltage. The pumping voltage here is regulated so that a constantvoltage of 450 mV is established on the electrodes of the concentrationcell. This voltage corresponds to a value of LAMBDA=1. Another electrodesituated in a second test gas space is operated with one of theseelectrodes as the second pumping cell. Because of the catalyticmaterial, this additional electrode functions as a NOx-sensitiveelectrode on which NOx is reduced according to the reaction NO→½N₂+{fraction (1/2 )}O ₂. At the same time, the above-mentioned referenceelectrode functions as the second pump electrode at which the oxygenpumped out of the second test gas space is released into the atmosphere.A limit current is thus established on the electrochemical cell, whichfunctions as an additional pumping cell, and this limit current ispicked up as a test signal which indicates the NOx concentration.

It should be pointed out that the diffusion barrier described in GermanPublished Patent Application No. 199 12 102 need not necessarily beincluded, and eliminating this barrier actually greatly reduces the gastravel times.

The measuring sensor described here may be used as a nitrogen oxide(NOx) sensor or as a hydrocarbon (HC) sensor. In its function as a NOxsensor, the NOx test signal reveals a dependence on the particularoxygen partial pressure in the measurement cell. This influence is duemainly to electric interactions of the sensor electrodes occurring inthe sensor ceramic. The main influence is based on the main pumpdistance shown in FIG. 1 between the external pump electrode (APE) andthe internal pump electrode (IPE), with the electric current (5 mA to 10mA) and thus its pumping level having to be taken into accountaccordingly at a variable oxygen content.

This O₂ influence is known to be compensated by electronic or computedaddition or subtraction with an IPE current-dependent factor usingsuitable analyzer circuits, the gain of this compensation having to beset specifically for each individual sensor. Such a circuit forelectronic compensation is shown in the block diagram in FIG. 1.

An object of the present invention is therefore to provide a method asdefined in the preamble and a circuit which avoid the disadvantagesmentioned above and minimize the above-mentioned influence of oxygen onthe nitrogen oxide signal using the simplest possible technical meansand the least expensive method possible.

The present invention is based on the idea of briefly suppressing thecause of the influence of oxygen on the nitrogen oxide signal, namelythe main pumping current prevailing between the internal pump electrode(IPE) and the external pump electrode (APE), so that an uncorrupted NOxsignal may be recorded during this period of time.

In a first variant according to the present invention, (main) pumpingcurrent I_pump is switched off, i.e., set at a value of 0, within ameasurement time window T_mess. In a second variant, an IPE currentcontrol sets the main pumping current at a constant value >0 duringmeasurement time window T_mess, so that although the influence of themain pumping current is not eliminated entirely, it is kept constantwhile the pumping level is reduced to a lesser extent and yet theamplitude of the oxygen concentration interference is reduced greatly.

In a preferred embodiment, measurement time window T_Mess is dimensionedso that the pumping current flowing between IPE and APE has alreadysubsided within T_Mess and the increase in oxygen concentration due tothe current switching off or current reduction has not yet reached theNOx electrode within T_Mess.

This above-mentioned intervention in the pumping current may beperformed regularly at a repeat frequency, whereby the repeat frequencyfor the current switching off or current reduction is of such adimension that the interference in the oxygen concentration has subsidedagain at the beginning of each subsequent IPE switching off orreduction. As an alternative, main pumping current I_pump may beswitched off or reduced temporarily during operation of the nitrogenoxide sensor and a calibration performed then.

According to one exemplary embodiment, measurement time window T_Mess isin the range of 10-100 μsec, preferably 60 μsec, and the repeatfrequency is in the range of 10-100 Hz, preferably 50 Hz.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the basic design of a main pump segment of a NOx sensoraccording to the related art.

FIG. 2 shows a circuit configuration according to the related art foroperation of the main pump segment illustrated in FIG. 1.

FIG. 3 shows an analyzer circuit according to the present invention forcompensation of the oxygen dependence of a NOx sensor.

FIG. 4 shows the circuit modifications illustrated in FIG. 3 for an IPEswitching off in another detail step.

FIG. 5 shows measured curves of uncompensated NOx signals and NOxsignals compensated by an IPE switching off according to the presentinvention for comparison.

FIG. 6 shows typical signal curves of pump voltage U_IPE and NOx signalU_NOx recorded at an IPE switching off according to the presentinvention.

DETAILED DESCRIPTION

FIG. 1 schematically shows the design of a main pump segment of a NOxsensor 10 known in the related art. Sensor 10 includes a first test gasspace 12 which is connected to the test gas (here: exhaust). A firstinternal pump electrode (IPE) 18 and a second internal pump electrode 20are situated in test gas space 12. Furthermore, a second test gas space14 is connected to first test gas space 12, a third electrode (ME) 22and a fourth electrode (NO) 24 being situated in this second test gasspace.

A reference gas channel 26 is situated independently of two test gasspaces 12, 14, leading out of the body of sensor 10 at one end andconnected to the atmosphere. Sensor 10 also has one or more gas inletopenings 28, which carry test gas into first test gas space 12.

Furthermore, an external electrode (APE) 30 which is situated on theexternal surface of a solid electrolyte layer (not shown) is exposeddirectly to the test gas. A fifth electrode (LR) 32 is also situated inreference gas channel 26, where it is exposed to the atmosphere; thiselectrode is referred to below as the air reference electrode. It shouldbe pointed out that alternatively fifth electrode 32 may be exposed tothe test gas.

In operation of the NOx sensor shown in FIG. 1, external electrode 30and first internal electrode 18 are operated as pump electrodes of afirst pumping cell. Second internal electrode 20 as a concentration cellis connected to fifth electrode 32, which functions as the referenceelectrode.

A pumping voltage U_APE,IPE is applied to electrodes 18, 30,establishing a constant oxygen partial pressure in first test gas space12 by pumping oxygen in or out. Pumping voltage U_APE,IPE applied toelectrodes 18, 30 is regulated so that a constant voltage of 450 mV, forexample, is set on electrodes 20, 32 of the concentration cell. Thisvoltage corresponds to LAMBDA=1.

With a lean test gas (LAMBDA>1), oxygen is pumped out of first test gasspace 12 by the first pumping cell. With a rich test gas (LAMBDA<1),oxygen from the test gas is pumped into first test gas space 12. Thechoice of electrode material and/or a suitable pumping voltage U_APE,IPEensures that no NOx is pumped out at electrodes 18, 20 when pumpingoxygen.

The test atmosphere, set at a constant oxygen partial pressure, goesthrough a connecting channel 16, which is indicated only schematically,into second test gas space 14. Third internal electrode (measuringelectrode ‘ME’) 22, which is in second test gas space 14, is operatedwith fifth electrode 30 as a second pumping cell. Because of thecatalytic material, fourth internal electrode 24 (here labeled as ‘NO’)acts as a NOx-sensitive electrode on which NOx is reduced according tothe reaction NO→½N₂+½O₂. At the same time, the reference electrodecooperating with electrode 20 acts as a second pump electrode at whichoxygen pumped out of second test gas space 14 is released into theatmosphere. A limit current indicating the NOx concentration is thusestablished on the electrochemical cell, which functions as the otherpumping cell, and is picked up as a test signal.

Furthermore, FIG. 1 illustrates the function of the main pump segment ofNOx sensor 10. The configuration shown here has the function of keepingthe oxygen concentration, i.e., the value of LAMBDA, at a constantvalue, e.g., LAMBDA=1, in first test gas space 12 on internal pumpelectrode 18. The value of LAMBDA in first test gas space 12 may beevaluated by the Nernst voltage occurring between internal pumpelectrode 18 and air reference electrode 32.

If internal pump electrode 18 is now set at ground potential 34, thevalue of LAMBDA occurring at internal pump electrode 18 may berepresented by the electric voltage between air reference electrode 32and ground 34. This value 35 forms the actual value of a controlledsystem 36 from which setpoint value U_lambda_setpoint 37 is subtractedby a summation element 38. The differential signal is sent to thenegative input of a two-position controller 40 designed as a differenceamplifier, in the present exemplary embodiment an operational amplifierhaving a gain >10,000. The reference input of two-position controller 40is at ground 42. Output 44 of two-position controller 40 is connected toexternal pump electrode 30. Oxygen is pumped into or out of first testgas space 12 according to the size and sign of the difference betweenU_LR 35 and U_lambda_setpoint 37.

FIG. 2 shows circuitry implementing the configuration shown in FIG. 1.To avoid corrupting voltage U_LR 35 measured on air reference electrode32 due to current flow, voltage U_LR 35 is transmitted via an isolationamplifier (OP3) 50 to summation point 38 at the negative input oftwo-position controller (OP5) 40. Setpoint value 37 is formed by anegative voltage, which is added to the actual value via a resistornetwork 52. Internal pump electrode 18 is applied to ground 56 via aguard amplifier 54. The output voltage of guard amplifier 54 representspumping current I_IPE occurring between external pump electrode (APE) 30and internal pump electrode (IPE), for which it thus holds thatI_IPE=U_IPE/R1.

FIG. 3 shows an analyzer circuit according to the present invention forcompensation of the oxygen dependence of a NOx sensor 10. This diagramshows that internal pump electrodes (IPE) 18, 20 are short-circuited,and together with external pump electrode (APE) 30 they are electricallyconnected via power supply lines to voltage supply terminals 100, 102 ofthe circuit shown. Corresponding terminals 104, 106 are provided formeasuring electrode (ME) 22 and NOx-sensitive electrode (NO) 24.

Furthermore, the analyzer circuit has a first circuit part 108 which isprovided for the basic equalization of sensor 10. Furthermore, a secondcircuit part 110, which is wired as an addition or subtraction stage, isalso provided and is responsible for the oxygen compensation describedabove via an IPE current-dependent factor which is provided by a linearconfiguration of operational amplifiers 112, 114, and 116. Theelectronics depicted in FIG. 3 are also connected electrically to airreference electrode 32 via a terminal 118.

It should be pointed out that the analyzer circuit depicted in FIG. 3also has some components that are known in the related art, inparticular the compensation stage formed by operational amplifiers 112through 116 for compensation of the O₂ influence with an IPEcurrent-dependent factor, as already mentioned in conjunction withFIG. 1. The broken-line diagram of parts 112 through 116 is intended toindicate that these parts are not necessary at the present time.

According to the present invention, the analyzer circuit has an IPEinterrupt module 404 connected via terminals 400, 402, and 102.Interrupt module 404 is also connected via a control line 406 to a NOxmeasured value acquisition module 408. IPE interrupt module 404interrupts main pumping current I_pump, i.e., sets it at a value ofzero, within a measurement time window T_Mess. As an alternative, it isalso possible for IPE interrupt module 404 to set the main pumpingcurrent at a constant value >0 during measurement time window T_Mess, sothat although the influence of the main pumping current is noteliminated entirely, it is kept constant while the pumping level isreduced to a lesser extent and yet the amplitude of the oxygenconcentration interference is reduced greatly.

Measurement time window T_Mess is dimensioned so that pumping currentI_pump flowing between IPE 18, 20 and APE 30 has already subsided withinT_Mess and the increase in oxygen concentration due to the currentswitching off or reduction has not yet reached NOx electrode 24 withinT_Mess.

In the exemplary embodiment, this change in pumping current I_pump isperformed at a repeat frequency, whereby the repeat frequency for thecurrent switching off or current reduction is of such a dimension thatthe interference in oxygen concentration has already subsided again atthe beginning of each subsequent IPE switching off or reduction.Measurement time window T_Mess in this exemplary embodiment is in therange of 10-100 μsec, preferably 60 μsec, and the repeat frequency is inthe range of 10-100 Hz, preferably 50 Hz.

As an alternative, it is possible to provide for main pumping currentI_pump to be switched off or reduced temporarily during operation ofnitrogen oxide sensor 10 and for a calibration to be performed thereby.This procedure may take place in conjunction with a digital signalprocessing by which a corresponding correction characteristics map iscalibrated in compensation or occasionally during operation. The latterprocedure may be implemented in the form of a self-learning system.

The NOx measured value acquisition module 408 shown here is used to beable to differentiate NOx values recorded within measurement time windowT_Mess and compensated with respect to oxygen from the uncompensatedmeasured values. To this end NOx measured value acquisition module 408is triggered in time via line 406 and outputs the measured data acquiredwithin measurement time window T_Mess via a separate output line 410provided for that purpose.

The upper half of FIG. 4 shows in greater detail IPE interrupt module404 described above and the bottom half of the figure shows in greaterdetail NOx measured value acquisition module 408, also describedpreviously. The function of IPE interrupt module 404 is to briefly rampdown main pumping current I_IPE and to do so as rapidly as possible toI_IPE=I_setpoint. I_setpoint=0 or I_setpoint>0. In this ramping down,the voltage proportional to current I_IPE on the resistor of guardamplifier 54 is picked up as the actual value acquisition, the output ofguard amplifier 54 being applied to terminal ‘2’ shown in FIG. 4 and theinput of guard amplifier 54 to terminal ‘3’ shown here. The voltagedifference applied to terminals ‘2’ and ‘3’ forms the input signal foran integral controller 500 which is wired as an interrupt regulator.

The output of integral controller 500 is connected via a switch 514 andan isolation amplifier 502 and via terminal ‘1’ is coupled to a resistor400 shown in FIG. 3 at summation point 38 of pumping current regulator40. Integral controller 500 here varies the voltage at summation point38 of pumping current regulator 40, so that pumping current I_IPE isregulated at I_setpoint after one oscillation process.

The process takes place during measurement time window T_Mess. First,switch 503 is closed and the output voltage of integral controller 500is 0 V. This ensures that at the beginning of the regulating process,the regulator intervention will have no effect on pumping currentregulator 40 and only integration in the direction predefined by I_IPEwill result in a defined transient phenomenon.

As an alternative, a switch may be provided in the line between externalpump electrode 30 and the regulator output of pumping current regulator40 by which pumping current I_IPE may be set at zero immediately, i.e.,without the transient phenomenon.

Furthermore, integral controller 500 may be designed so that the timeconstant for integration is switchable and, for example, may be switchedbetween a relatively short first time constant and a relatively longsecond time constant in comparison with the first time constant. Thefirst time constant may thus be set just before a zero crossing ofpumping current I_IPE and only then switched to the second timeconstant, resulting in an aperiodic transient phenomenon.

Before the start of T_Mess, switch 514 is opened. Resistor 510 ensuresthat no voltage is supplied at summation point 38 during regularregulator operation and thus there is no effect on the currentregulation. At the start of measurement time T_Mess, switch 503 isopened and switch 514 is closed so that the intervention measure ofintegral controller 500 is able to act on pumping current regulator 40.

NOx measured value acquisition 408 shown in the lower half of FIG. 4 hasthe function of detecting and storing the NOx signal at the point intime of a defined set pumping current I_IPE=I_setpoint.

The circuit configuration shown here contains a circuitry implementationof the NOx determination method depicted in FIG. 2. The signal generatedthereby is used further in the IPE interrupt method according to thepresent invention without any technical changes in the circuit, theinfluence of the potentiometers (not shown), which are known in therelated art and are provided separately for that purpose in thescattering current compensation shown in FIG. 3, being set at zero. Thefunctions of the three amplifier stages 112, 114, and 116 (marked out inFIG. 3) shown in FIG. 3 therefore have practically no effect.

The NOx measured value acquisition is controlled in synchronization withintegral controller 500. First a switch 526 is opened. A holding elementcomposed of a capacitor 524 and an isolation amplifier 520 has an outputvoltage (terminal ‘13’) which corresponds to the instantaneous chargestate of capacitor 524. During measurement time T_Mess, switch 526 isclosed so that capacitor 524 is recharged according to the input voltageapplied to terminal ‘12’. Terminal ‘12’ is connected to the NOx signaloutput shown in FIG. 3 of the analyzer circuit there.

At the end of measurement time T_Mess, switch 524 is opened again, sothat the value of the NOx signal established at I_IPE=I_setpoint isstored.

FIG. 5 shows a comparison of measured curves of an uncompensated NOxsignal and NOx signals sampled during a measurement time window T_Messand compensated according to the present invention, namely with variableO₂ concentrations and variable NOx concentrations. A NOx signal recordedis plotted as a function of time in this diagram. If the value of theNOx signal is also recorded and stored just before the start of themeasurement time, then the compensated and uncompensated curves may becompared.

However, it should be pointed out that the flanks of the O₂concentration change are not compensated by the method according to thepresent invention.

FIG. 6 shows the signal curves of U(I_IPE) and U(I_NOx) during themeasurement time with the sampling points in time for the compensatedand uncompensated signal, where U(I_IPE) and U(I_NO) denote the outputvoltages, which are applied to electrodes ‘IPE’ 18 and ‘NO’ 24, of guardamplifiers 54, 55 described above. Furthermore, U(IPE) denotes thevoltage applied at the positive input of guard amplifier 54 at IPE.

1-12. (Canceled)
 13. A method for operating a nitrogen oxide sensor fordetermining a nitrogen oxide concentration in a gas mixture, comprising:applying an electric pumping voltage that induces a first pumpingcurrent between an internal pump electrode and an external pumpelectrode of a pumping cell, by which a constant oxygen partial pressureis set in a first test gas space by pumping oxygen in or out; regulatingthe pumping voltage so that a constant voltage value is established atelectrodes of a concentration cell; operating an NOx-sensitive thirdelectrode situated in a second test gas space as a second pumping cellin which a limit pumping current is established, indicating the NOxconcentration; one of switching off and reducing in a controlled mannerthe first pumping current within a measurement time window; andrecording the NOx concentration within the measurement time window. 14.The method as recited in claim 13, wherein: the gas mixture includes anexhaust-gas aftertreatment of a motor vehicle.
 15. The method as recitedin claim 13, wherein: the measurement time window is dimensioned so thatthe first pumping current flowing between the internal pump electrodeand the external pump electrode subsides within the measurement timewindow and an increase in an oxygen concentration created due to thefirst pumping current being one of switched off and reduced does notreach the Nox-sensitive third electrode within the measurement timewindow.
 16. The method as recited in claim 13, wherein: the firstpumping current is one of switched off and reduced at a repeatfrequency, whereby the repeat frequency is of such a dimension that aninterference in an oxygen concentration subsides at a beginning of eachsubsequent one of switching off and reduction.
 17. The method as recitedin claim 16, wherein: the measurement time window is in the range of10-100 μsec, and the repeat frequency is in the range of 10-100 Hz. 18.The method as recited in claim 16, wherein: the measurement time windowis 60 μsec, and the repeat frequency is 50 Hz.
 19. The method as recitedin claim 12, wherein: the first pumping current is one of switched offand reduced temporarily during an operation of the nitrogen oxide sensorand a calibration is performed thereby.
 20. A circuit for operating anitrogen oxide sensor for determining a nitrogen oxide concentration ina gas mixture, comprising: an arrangement for applying an electricpumping voltage that induces a first pumping current between an internalpump electrode and an external pump electrode of a pumping cell, bywhich a constant oxygen partial pressure is set in a first test gasspace by pumping oxygen in or out; an arrangement for regulating thepumping voltage so that a constant voltage value is established atelectrodes of a concentration cell; an arrangement for operating anNOx-sensitive third electrode situated in a second test gas space as asecond pumping cell in which a limit pumping current is established,indicating the NOx concentration; an arrangement for one of switchingoff and reducing in a controlled manner the first pumping current withina measurement time window; and an arrangement for recording the NOxconcentration within the measurement time window.
 21. The circuit asrecited in claim 20, further comprising: an IPE current control thatsets the first pumping current one of at zero and at a constant valuegreater than zero during the measurement time window.
 22. The circuit asrecited in claim 21, wherein: the IPE current control sets themeasurement time window in the range of 10-100 μsec and the repeatfrequency in the range of 10-100 Hz.
 23. The circuit as recited in claim21, wherein: the IPE current control sets the measurement time window to60 μsec and the repeat frequency to 50 Hz.
 24. The circuit as recited inclaim 21, further comprising: a connecting line; a two-positioncontroller connected to the external pump electrode; and a switchsituated in the connecting line between the external pump electrode anda regulator output of the two-position controller.
 25. The circuit asrecited in claim 24, further comprising: an integral controllerconnected upstream from the two-position controller and situated in theIPE current control.
 26. The circuit as recited in claim 25, wherein:the integral controller includes an arrangement for setting a timeconstant for an integration.
 27. The circuit as recited in claim 26,wherein: the integral controller is operated using a first time constantuntil just before a zero crossing of the pumping current and thereafteris operated using a second time constant that is larger than the firsttime constant.